The invention relates to a device for contactless temperature measurement of an object with an optical system which images into the finite and a detector. In contactless temperature measurement it is indispensable in practical use that the measurement spot on the object to be examined should be rendered visible in a suitable way. U.S. Pat. No. 5,368,392 proposes the deflection of a laser beam onto a rotating deflecting mirror to draw a circle representing the measurement spot on the object. However, this sighting arrangement can only used in an optical system which images into infinity. In an optical system which images into the finite according, an image of the detector is reduced and then enlarged by the optical system along an optical axis onto a measurement spot on the object from the optical system to the so-called sharp point measurement spot. In the past, in order to identify the measurement spot in an optical system which images into the finite, an opening laser cone according to U.S. Pat. No. 5,368,392 was used for example, which has its origin in the optical system on the optical axis and intersects the actual beam path of the optical system once before and once after the sharp point measurement spot. Thus, apart from the two planes of intersection, the sighting spot thus produced merely constitutes an approximate representation of the actual measurement spot.
Another possibility consists of directing individual sighting rays from the outer periphery of the optical system onto the point of intersection of the optical axis and the sharp point measurement spot. The resulting sighting measurement spot reduces to 0 up to the sharp point measurement spot and then enlarges again. This solution also only shows the approximate size of the measurement spot.
In order to reproduce a more precise course of the beam path of the optical system it has also been proposed to provide a beam for the region between the optical system and the sharp point measurement spot and a further beam for the region thereafter. However, this means that in each case two sighting measurement spots of differing size, caused by the two sighting beams, are visible both before and after the sharp point measurement spot. This in turn means on the one hand that the allocation of the correct sighting beam presents difficulties in certain circumstances and on the other hand the light energy necessary for producing the sighting light must be divided over the two beams. Accordingly the intensity of the sighting measurement spot is also correspondingly lower.